Method and apparatus for mixing gases

ABSTRACT

A method and apparatus for mixing a first stream of gas with a second stream of gas is provided. The method includes introducing a first stream of gas into a first stream manifold and directing the first stream from the first stream manifold into a mixing chamber via a plurality of first stream passages flow coupled to the mixing chamber. A second stream of gas is directed into the mixing chamber via at least one second stream passage flow coupled to a first end of the mixing chamber. A combined stream is formed from the first and second streams, gradually converged, and discharged from the mixing chamber through a mixing chamber exit port.

TECHNICAL FIELD

[0001] The present invention is directed to a method and an apparatusfor mixing streams of gases to form a combined stream.

BACKGROUND

[0002] The federal government and other authorities regulate allowableexhaust emissions from gasoline and diesel engines for automobiles,trucks, and other vehicles, such as off-road construction oragricultural vehicles, in an effort to reduce pollution. In order toensure compliance with these regulations, the exhaust gases of theseengines must be tested or otherwise analyzed for undesirable combustionby-products, such as hydrocarbons, carbon monoxides, sulphates, and/oroxides of nitrogen. In general, testing is accomplished by introducingexhaust gases, diluting these exhaust gases with clean air, andobtaining samples after the exhaust gases and dilution air are properlymixed.

[0003] Dilution tunnels are one known type of device for collecting,diluting, cooling, and mixing exhaust gases with filtered, ambient, andconditioned air in a ratio of gas to air for sampling and analyzing. Atone end, a typical dilution tunnel has one inlet for receiving exhaustgases and another inlet for receiving the filtered air. An orifice plateis typically placed downstream of the exhaust gas and air inlets inorder to induce turbulent flow and facilitate mixing of the exhaust gasand diluting air. Downstream of the orifice plate a probe is located inthe tunnel to collect a sample of the mixture for analysis. One problemwith placing orifice plates, or other obstructions such as inlet pipes,in the stream is that they tend to collect and remove some of theparticulate matter from the stream, which distorts the downstreamsamples. Moreover, abrupt changes in tunnel geometry may also causeparticulate matter to collect on the tunnel walls. This particulatematter may build up on the plate or walls over time and thenperiodically flake off, further distorting the samples taken downstream.

[0004] Full dilution tunnels collect and dilute the entire exhaust gasflow from the engine being tested. Current EPA regulations recommendthat the dilution tunnels be sized to permit development of turbulentflow (Reynold's number greater than 4000) and obtain a homogeneousmixture of the exhaust and dilution air at the sampling location.Depending upon the engine displacement, in order to meet thisrequirement, a typical full dilution tunnel diameter may be on the orderof 203 to 610 mm (8 to 24 inches) and a typical tunnel length, which isusually ten times the diameter, may be on the order of 2032 to 6100 mm(80 to 240 inches). The tunnel diameter and length is sized to insureproper mixing of the exhaust gas with the diluting air prior to takingthe sample. The larger the engine displacement, the larger the tunneldiameter and tunnel length must be to accommodate the greater flow ofexhaust gas. Thus, dilution tunnels for large displacement engines maybe very bulky, even taking up entire rooms.

[0005] U.S. Pat. No. 5,090,258 discloses a multiple flow-dividingdilution tunnel system. Dilution air is introduced at one end of thetunnel. Further downstream, a portion of an exhaust gas stream isintroduced into the tunnel via an exhaust gas inlet pipe inserted intothe stream of dilution air. Further downstream, but proximate the end ofthe exhaust gas inlet pipe, a plurality of nozzles are inserted into thestream of dilution air/exhaust gas for introducing more dilution air, inthis instance, a controlled amount of pressurized dilution air. Thisintroduction of pressurized dilution air is used to properly maintainthe ratio of the rate of the divided exhaust gas introduced into thedilution tunnel via the exhaust gas inlet pipe to the rate of the totalgas introduced into the system, even during pressure loss fluctuations.One drawback of this system is that it requires a complex control systemfor reacting to pressure fluctuations. Another drawback is that it alsorequires a complex system of piping, both for splitting the exhaust gasstream and for introducing the pressurized dilution air. A furtherdrawback is that the exhaust inlet pipe is exposed to the dilution airstream prior to the exhaust gas exiting the inlet pipe, causingundesirable cooling of the exhaust gas prior to mixing.

[0006] The disclosed method and apparatus for mixing streams of gasessolves one or more of the problems set forth above.

SUMMARY OF THE INVENTION

[0007] One aspect of the present invention is directed to a method ofmixing a first stream of gas with a second stream of gas. The methodincludes introducing the first stream of gas into a first streammanifold and directing the first stream from the first stream manifoldinto a mixing chamber via a plurality of first stream passages flowcoupled to the mixing chamber. The second stream of gas is directed intothe mixing chamber via at least one second stream passage flow coupledto a first end of the mixing chamber. A combined stream is formed fromthe first and second streams, gradually converged, and discharged fromthe mixing chamber through a mixing chamber exit port.

[0008] In another aspect, the present invention is directed to anapparatus for mixing a first and second stream of gas. The apparatusincludes a first stream manifold configured to receive the first streamof gas, and a first plurality of passages flow coupled to and extendingfrom the first stream manifold. A mixing chamber having first and secondends is flow coupled to the first plurality of passages and configuredto receive the second stream of gas at the first end. The mixing chamberhas an exit port at the second end and a cross-section adjacent thesecond end which gradually converges as the distance to the exit portdecreases.

[0009] It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory only and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The accompanying drawings, which are incorporated in andconstitute a part of this specification, illustrate exemplaryembodiments of the invention and together with the description, serve toexplain the principles of the invention. In the drawings:

[0011]FIG. 1 is a schematic and diagrammatic illustration of anembodiment of an apparatus for mixing gases;

[0012]FIG. 2 is schematic cross-sectional illustration of a portion ofthe apparatus of FIG. 1 rotated 45 degrees so that at least two passages14 and passage 24 lie in the viewing plane;

[0013]FIG. 3 is a cross-section of the apparatus of FIG. 2 taken atsection A-A;

[0014]FIG. 4 is a schematic and diagrammatic illustration of anotherembodiment of an apparatus for mixing gases; and

[0015]FIG. 5 is schematic and diagrammatic illustration of a furtherembodiment of an apparatus for mixing gases.

DETAILED DESCRIPTION

[0016]FIG. 1 shows an apparatus, designated generally by referencenumber 100, which includes a mixing chamber 30 receiving a first stream10 and a second stream 20. In one embodiment, first stream 10 may beeither a stream of exhaust gas or a stream of dilution gas, and secondstream 20 may be the other of the stream of exhaust gas or dilution gas.For example, first stream 10 may be a stream of dilution gas and secondstream 20 may be a stream of exhaust gas. Thus, mixing chamber 30, whichreceives both first stream 10 and second stream 20, would receive, inthis embodiment, both the stream of exhaust gas and the stream ofdilution gas. The stream of exhaust gas, which may be generated by anengine, such as a diesel engine, contains particulate matter and/orother emissions that are to be measured. The stream of dilution gas maybe conditioned air, i.e., air that is filtered, heated, cooled,humidified, de-humidified, etc.

[0017] In one embodiment, as shown in FIG. 1, apparatus 100 may include,among other components, first stream manifold 12, passages 14, 24,mixing chamber 30, and secondary mixing region 50. Each of thesecomponents may be designed and manufactured with no internalprotuberances or projections that could collect particulate matter, ifany, carried by the first and/or second streams 10, 20. Moreover, theinternal wall surfaces of each of these components may be made of smoothmaterials, such as an electro-polished, passivated, stainless steel orother smooth, relatively non-stick materials, for minimizing the amountof particulate matter that could collect on the walls. In addition,these components may be assembled in a manner that minimizes the numberof wall surfaces upon which the first and second streams 10, 20 impingeor that minimizes the angle of impingement.

[0018] As shown in FIGS. 1 and 2, prior to entering mixing chamber 30,first stream 10 travels through an inlet pipe 11 and enters first streammanifold 12. First stream manifold 12 is used to evenly distribute firststream 10 into a plurality of passages 14. Passages 14 extend from firststream manifold 12 to mixing chamber 30 and are used to evenly introducethe flow of first stream 10 into mixing chamber 30. Additionally,passages 14 may be sized and configured to allow a well-developed flowto develop within first stream 10 as it flows through these passages. Awell-developed flow has a steady state mean velocity profile, i.e., amean velocity profile that does not change with distance in the flowdirection. If a straight line passes through the diameter of awell-developed flow, the velocity at a each point of the line would bedifferent. The velocity near the walls would be a minimum velocity, forinstance, a zero velocity, while the velocity near the center of thepassage would be a maximum. A well-developed flow generally arises in along pipe if the flow is not subject to any protrusions, changes incross-section, or other disturbances. For example, in FIG. 1, passages14 are depicted as being smooth-walled, relatively thin, straight tubessufficiently long to develop a well-developed turbulent flow prior todischarging first stream 10 into mixing chamber 30.

[0019] Second stream 20 is shown in FIGS. 1 and 2 entering mixingchamber 30 through second stream passage 24. Similar to passages 14,second stream passage 24 may be sized and configured to allow aturbulent flow to form within second stream 20 as it flows throughpassage 24. Passage 24 may be sized and configured such that, prior toentering mixing chamber 30, the flow of second stream 20 is awell-developed turbulent flow, with no separation at the passage walls.Passage 24 may be a smooth-walled, relatively long, narrow tube having,for example, two rounded ninety-degree bends.

[0020] First stream 10 and second stream 20 are introduced into mixingchamber 30. Mixing chamber 30 has a first end 31 and a second end 32opposite first end 31. As shown in FIGS. 2 and 3, first end 31 may be aflat wall. Both first and second streams 10, 20 are introduced intomixing chamber 30 at first end 31. As best shown in FIG. 2, passages 14and passage 24 do not extend or protrude into mixing chamber, but endsubstantially flush with the wall of first end 31. This flushconfiguration allows for even mixing of first stream 10 with secondstream 20, and ensures that neither the first stream nor the secondstream is in contact with the passages of the other prior to dischargingthe streams into the mixing chamber. Isolating the first stream passagefrom the second stream prevents the second stream from prematurelyand/or unevenly heating or cooling the first stream. Moreover, isolatingthe two streams from one another prior to their being discharged intomixing chamber 30 prevents the streams from becoming non-uniform orbiased prior to discharge. For example, if the first stream was a hotexhaust gas stream and the second stream was a cool air stream, thencontact between the passages of the hot exhaust gas stream and the coolair stream could cause the undesirable deposition of soot or otherparticulate matter on the walls of the exhaust gas stream passage due tothermophoretic effects. In addition, apparatus 100 or individualcomponents of apparatus 100 may be insulated from the surroundingevironment and/or from one another to assist in minimizingthermophoretic effects.

[0021] As best shown in FIG. 3, the plurality of passages 14, whichdischarge first stream 10 into mixing chamber 30, are symmetricallypositioned with respect to the cross-section of mixing chamber 30 andwith respect to passage 24, which discharges second stream 20 intomixing chamber 30. Four passages 14 are shown, but any number of aplurality of passages may be utilized. A single passage 24 is shown,located in the center of first end 31, for discharging second stream 20into mixing chamber 30. In alternative embodiments, more than onepassage 24 may be provided. The symmetrical and distributed discharge offirst stream 10 into mixing chamber 30 aids in the uniform and efficientmixing of first stream 10 with second stream 20.

[0022] Mixing chamber 30, as best shown in FIG. 2, is a smooth-walledchamber having no internal protuberances or projections, which couldcollect particulate matter, if any, carried by the first and/or secondstream 10, 20. Moreover, the internal wall surfaces 36 of mixing chamber30 may be formed of a very smooth surface for minimizing the amount ofparticulate matter that collects on the walls. For instance, theinternal wall surfaces 36 may be electro-polished, passivated, stainlesssteel or other smooth, relatively non-stick surfaces.

[0023] Within mixing chamber 30, first stream 10 and second stream 20lose their separate identities and are combined into a combined stream40. Combined stream 40 exits mixing chamber 30 through an exit port 34located at second end 32. Combined stream 40 need not be a uniformlymixed stream by the time it exits mixing chamber 30 through exit port34. Rather, combined stream 40 may include both first and second streams10, 20 in a partially mixed state.

[0024] Second end 32 of mixing chamber 30 may include a portion whereinthe cross-section gradually and symmetrically converges or necks down asthe distance to exit port 34 decreases. A cross-section “graduallyconverges” when the cross-section becomes smaller and smaller in aseries of steps or degrees, as opposed to an abrupt transition betweentwo differently sized openings. As shown in FIGS. 1 and 2, theconverging portion of second end 32 has the shape of a truncated cone.This converging portion may be part of a venturi-type nozzle. Thegradual convergence of the walls of second end 32 provides a smooth,rather than an abrupt, transition to the reduced diameter of exit port34. This smooth, gradual convergence is intended to prevent particlesfrom collecting on the walls of end 32. Moreover, the symmetricalconfiguration of second end 32, as it converges, assists in the uniformmixing of combined stream 40.

[0025] As best shown in FIG. 1, downstream of mixing chamber 30,combined stream 40 may enter a secondary mixing region 50. Secondarymixing region 50 is depicted in FIG. 1 as having a short initial section51 located immediately downstream of mixing chamber 30 and a longercylindrical section 52 located downstream of section 51. Thecross-sectional diameter of section 52 is greater than thecross-sectional diameter of section 51. An abrupt enlargement of thecross-sections, as shown in FIG. 1, may be advantageous because ofmanufacturing considerations. The abrupt enlargement may also enhancemixing.

[0026] Within secondary mixing region 50, uniform mixing, such asrequired for testing, of combined stream 40 may be achieved. A samplingdevice (not shown), such as any probe known to persons of ordinary skillin the art, may be located in secondary mixing region 50, such as, forexample, adjacent to a downstream end thereof.

[0027] Downstream of secondary mixing region 50, combined stream 40 maybe directed through a turn, for example, around a corner, to minimizethe total length of the system. However, turning the stream may induceundesirable separation effects in combined stream 40 upstream of theturn, i.e., where the sampling probe may be located. A reservoir box 60,which may be a relatively large-volume chamber, may be flow coupled tosecondary mixing region 50 at the downstream end of region 50. As shownin FIG. 1, combined stream 40 exits secondary mixing region 50 and isdischarged into reservoir box 60. Reservoir box 60 provides a pressuredrop in the downstream flow and serves to reduce any flow separationeffects that might be induced in combined stream 40 adjacent thedownstream end of secondary mixing region 50, while at the same timedirecting combined stream through a turn. Reservoir box 60 may be anychamber, including, for instance a porous baffle (not shown), thatprovides a pressure drop that reduces undesirable flow separationeffects. Combined stream 40, as shown in FIG. 1, is then discharged fromthe system.

[0028] A second embodiment of an apparatus for mixing gases isillustrated in FIG. 4 and is also designated generally by referencenumber 100. As shown in FIG. 4, apparatus 100 includes a mixing chamber30, a first stream manifold 12, a second stream manifold 22, and asecondary mixing region 50.

[0029] First stream manifold 12 may be configured as an annular chamberencircling mixing chamber 30, as shown in FIGS. 4 and 5. FIG. 4illustrates an embodiment which is configured to receive two alternativefirst streams 10 a, 10 b. In this embodiment, first stream manifold 12may receive either first stream 10 a or first stream 10 b throughentrance ports 13 a, 13 b, respectively, located on opposite sides ofannular manifold 12. Alternatively, the embodiment illustrated in FIG. 5is configured to receive only one first stream, first stream 10. Asshown in FIG. 5, first stream manifold 12 has only a single entranceport 13 for receiving first stream 10.

[0030] Entrance ports 13, 13 a, 13 b may include diffuser sections forslowing the flow of first streams 10, 10 a, 10 b, respectively. Diffusersections may inhibit the impingement of first streams 10, 10 a, 10 b onthe wall of first stream manifold 12 opposite the entrance ports 13, 13a, 13 b, thereby, for instance, inhibiting any particulate matter thatmay be carried by streams 10, 10 a, 10 b from collecting or beingdeposited on the wall of first stream manifold 12. First stream manifold12 may be formed with smooth internal wall surfaces to assist inminimizing the deposition of particulate matter on the walls.

[0031] A plurality of first stream passages 14 are flow coupled to andextend between first stream manifold 12 and mixing chamber 30. As shownin FIGS. 4 and 5, passages 14 extend radially between annular manifold12 and centrally located mixing chamber 30. In this configuration,passages 14 allow first stream 10 to enter mixing chamber 30 adjacentfirst end 31. Passages 14 need not lie in a plane perpendicular to acentral, longitudinal axis of the annular manifold and the mixingchamber. Rather, as shown, passages 14 may be angled out of thisperpendicular plane.

[0032] Mixing chamber 30, as best shown in FIG. 5, may have a first end31, having an entrance port 35 and walls which gradually diverge as thedistance from entrance port 35 increases. Mixing chamber 30 may alsohave a second end 32, having an exit port 34 and walls which graduallyconverge as the distance from exit port 34 decreases.

[0033] Second stream 20 enters second stream manifold 22 prior toentering mixing chamber 30 via entrance port 35. Second stream manifold22 may be an annular manifold similar to annular manifold 12 as shown inFIGS. 4 and 5, or second stream manifold 22 may be similar to firstmanifold 12 as shown in FIGS. 1 and 2. As shown in FIGS. 4 and 5, secondstream 20 is discharged from annular manifold 22, via passages 24 into acentral tubular region 25, which is flow coupled to entrance port 35 ofmixing chamber 30.

[0034] Within mixing chamber 30 of FIGS. 4 and 5, as with mixing chamber30 of FIGS. 1 and 2, first stream 10 and second stream 20 lose theirseparate identities and are combined into a combined stream 40, whichexits mixing chamber 30 through exit port 34. Secondary mixing region 50is flow coupled to mixing chamber 30 and receives combined stream 40. Asshown in FIG. 4, secondary mixing region 50 gradually diverges as thedistance from exit port 34 increases. This gradual divergence causes agradual pressure drop in combined stream 40. Adjacent the downstream endof secondary mixing region 50, a sampling device (not shown) may belocated in order to collect samples of a fully mixed combined stream 40.

INDUSTRIAL APPLICABILITY

[0035] The Environmental Protection Agency (EPA) requires transientemissions tests to be conducted for on-highway diesel engines. Whenutilizing a full dilution tunnel system, the EPA regulations (forinstance, 40 CFR, Chapter 1, §83.1310) require that the raw dieselengine exhaust be mixed or diluted with clean air in order to reduce theoverall temperature of the exhaust levels. These EPA regulations requiregood mixing over the entire cross-section of the diluted flow of exhaustat the point where the emission samples are collected.

[0036] The apparatus and method provided may be used for mixing a firstgas with a second gas prior to sampling a combined stream of the gases,in order to, for example, efficiently test exhaust gas samples forcompliance with the EPA pollution regulations. In one exemplary use, theexhaust pipe of a diesel engine could be flow coupled to passage 24 asshown in FIG. 1. Exhaust gases from the diesel engine flow throughpassage 24, becoming a well-developed flow, which is introduced intomixing chamber 30. Minimal wall-separation of the exhaust flow isachieved, for instance, by providing two ninety-degree bends in passage24 in three-dimensions. These bends allow the exhaust stream to beintroduced into mixing chamber 30 in a minimal space, while at the sametime allowing the exhaust stream flow to straighten and become awell-developed flow very quickly.

[0037] A source of conditioned dilution air could be flow coupled toinlet pipe 11, also as shown in FIG. 1. The dilution air flows intomanifold 12 and then is distributed through the four passages 14 intomixing chamber 30. As the dilution air travels through passages 14, awell-developed flow is created in each passage, prior to beingintroduced into mixing chamber 30.

[0038] Passage 24, which carries the exhaust gas, is never in contactwith the dilution air stream. Similarly, the exhaust gas stream nevercontacts passages 14, which carry the dilution air. Isolating thesepassages from the opposing streams prevents the possibility ofpre-cooling the exhaust stream prior to its discharge into mixingchamber 30, which could lead to soot deposition on the walls of passage24, and also prevents the possibility of the streams losing theirwell-developed flow profiles. Moreover, isolating these passages fromthe opposing streams greatly simplifies insulation concerns.

[0039] Apparatus 100 includes components, for instance, passages 14,manifold 12, passage 24, mixing chamber 30, secondary mixing region 50,etc., that have smooth internal walls, i.e., walls having surfaces thatare not rough, to minimize the deposition of particulate matter on theinternal wall surfaces. In addition, these components are free of anyinternal projections, such a inlet pipes, or other structures, such asbaffles or orifice plates, extending from the internal walls into theflows. So, for instance, minimal particulate matter from the exhaust gasis deposited on surfaces within mixing chamber 30, because mixingchamber 30 has no projections, edges, or other structure extending intothe flow stream. Moreover, if smooth, electro-polished, passivated,stainless steel is used to form the internal surfaces of mixing chamber30 or other components of apparatus 100, the deposition of particulatematter on the internal surfaces of apparatus 100 may be minimized.

[0040] Within mixing chamber 30, the exhaust gas and the dilution airbegin to mix in a uniform manner into a combined stream 40. Prior toexiting mixing chamber 30 through exit port 34, combined stream 40travels through a converged portion of mixing chamber 30. This convergedor necked down portion of mixing chamber 30 causes the velocity and thepressure of combined stream 40 to increase, and facilitates quick anduniform mixing of the combined stream. Downstream of mixing chamber 30,combined stream 40 travels through a cylindrical, venturi type, orsimilar type of secondary mixing region 50 to finalize the uniformmixing of the exhaust gas with the dilution air prior to a portion ofthe combined stream being collected for sampling purposes.

[0041] Even for large output engines, such as engines rated between 150and 750 horsepower, apparatus 100, because of its compact size relativeto existing full dilution tunnels, may be designed to fit into the sameroom that also houses the engine. For instance, apparatus 100 may bedesigned with pipes being no more than 0.64 meters (24 inches) indiameter. Finally, apparatus 100 may be designed to allow the use ofstandard pipes and standard-pipe fittings, thus, minimizing the use ofspecially built parts and substantially decreasing the cost ofmanufacture.

[0042] The disclosed system may also be utilized with dual test cells,thereby providing an even more compact and efficient system. As anexample of the use of the embodiment shown in FIG. 4, a first testchamber (not shown) may be located adjacent apparatus 100 and maycontain a first stream source (not shown). For instance, a diesel engineemitting a first stream 10 a of an exhaust gas may be the first streamsource. This first stream source may be flow coupled to entrance port 13a. A second test chamber (not shown) with a second stream source (notshown) may be located on the other side of apparatus 100. Second streamsource may be a second diesel engine emitting a stream of exhaust gas asalternative first stream 10 b. This second exhaust gas source producingalternative first stream 10 b may be flow coupled to entrance port 13 b.Thus, the embodiment with two entrance ports, shown in FIG. 4, mayservice two separate test chambers. In a first test, entrance port 13 amay be opened, allowing first stream 10 a to flow into manifold 12,while entrance port 13 b may be closed, blocking the flow of alternativefirst stream 10 b into manifold 12. In a subsequent test, entrance port13 b may be opened and entrance port 13 a may be closed.

[0043] It will be apparent to persons of ordinary skill in the art thatvarious modifications and variations can be made in the method andapparatus for mixing gases of the present invention without departingfrom the scope or spirit of the invention. Additionally, otherembodiments of the invention may be apparent to persons of ordinaryskill in the art from consideration of the specification and practice ofthe invention disclosed herein. It is intended that the specificationand examples be considered as exemplary only, with a true scope andspirit of the invention being indicated by the following claims andtheir equivalents.

What is claimed is:
 1. A method of mixing a first stream of gas with asecond stream of gas, comprising: introducing a first stream of gas intoa first stream manifold; directing the first stream from the firststream manifold into a mixing chamber via a plurality of first streampassages flow coupled to the mixing chamber; directing a second streamof gas into the mixing chamber via at least one second stream passageflow coupled to a first end of the mixing chamber; forming a combinedstream from the first and second streams; gradually converging thecombined stream; and discharging the converged combined stream from themixing chamber through a mixing chamber exit port.
 2. The method ofclaim 1, further including: expanding the converged combined streamdownstream from the exit port.
 3. The method of claim 1, furtherincluding: introducing the second stream into a second stream manifold,and wherein directing the second stream includes directing the secondstream from the second stream manifold via a plurality of second streampassages flow coupled to the mixing chamber.
 4. The method of claim 1,wherein the first stream is directed into the mixing chamber at thefirst end of the mixing chamber.
 5. The method of claim 1, furtherincluding: developing a substantially well-developed flow of the firststream within the plurality of first stream passages.
 6. The method ofclaim 5, further including: developing a substantially well-developedflow of the second stream within the at least one second stream passage.7. The method of claim 1, wherein the first stream is one of a stream ofdilution air and a stream of exhaust gas from an engine, and the secondstream is the other of the stream of dilution air and the stream ofexhaust gas from an engine.
 8. The method of claim 7, further including:sampling the combined stream for compliance with emission standards. 9.An apparatus for mixing a first and a second stream of gas, comprising:a first stream manifold configured to receive the first stream of gas; afirst plurality of passages flow coupled to and extending from the firststream manifold; and a mixing chamber having first and second ends, themixing chamber flow coupled to the first plurality of passages andconfigured to receive the second stream of gas at the first end, themixing chamber having an exit port at the second end and a cross-sectionadjacent the second end which gradually converges as the distance to theexit port decreases.
 10. The apparatus of claim 9, further including: asecondary mixing region flow coupled to the mixing chamber downstream ofthe exit port, wherein the secondary mixing region has one of across-section that gradually increases as the distance from the exitport increases and a cross-section that abruptly increases.
 11. Theapparatus of claim 10, further including: a second stream passage flowcoupled to the mixing chamber at the first end and adapted to dischargethe second stream of gas into the mixing chamber, wherein the firststream manifold, the plurality of passages, the second stream passage,the mixing chamber, and the secondary mixing region have smooth walls,and wherein at least the second stream passage, the mixing chamber, andthe secondary mixing region are insulated.
 12. The apparatus of claim 9,further including: a secondary mixing region flow coupled to the mixingchamber downstream of the exit port; and a reservoir box flow coupled tothe secondary mixing region at an end opposite the end which is flowcoupled to the exit port.
 13. The apparatus of claim 9, wherein themixing chamber has internal wall surfaces formed of electro-polished,passivated, stainless steel.
 14. The apparatus of claim 9, wherein themixing chamber has smooth internal walls and no projections extendinginwardly from the internal walls.
 15. The apparatus of claim 9, furtherincluding: a second stream manifold flow coupled to the second stream ofgas; and a second plurality of passages flow coupled to and extendingbetween the second stream manifold and the mixing chamber, wherein thefirst and second plurality of passages do not substantially extend intothe mixing chamber.
 16. The apparatus of claim 15, wherein the first andsecond stream manifolds are annular chambers.
 17. The apparatus of claim9, wherein the first plurality of passages are flow coupled to themixing chamber at the first end of the mixing chamber.
 18. A method ofmixing an exhaust gas with a dilution gas, comprising: providing a firstgas which is one of an exhaust gas and a dilution gas; providing asecond gas which is the other of an exhaust gas and a dilution gas;providing a mixing chamber having a first end and a second end andsubstantially smooth, unobstructed, internal walls therebetween;introducing at least one substantially well-developed flow stream of thesecond gas into the mixing chamber at the first end; introducing aplurality of substantially well-developed flow streams of the first gasinto the mixing chamber in a pattern that is symmetric with respect toboth a cross-section of the mixing chamber and the at least one streamof the second gas introduced into the mixing chamber; and discharging acombined stream of the first gas and the second gas from the mixingchamber through a convergent cone region located at the second end ofthe mixing chamber into a secondary mixing region.
 19. The method ofclaim 18, further including: directing the first gas into a first gasmanifold chamber prior to introducing the plurality of substantiallywell-developed flow streams of the first gas into the mixing chamber.20. The method of claim 19, further including: directing the second gasinto a second gas manifold chamber prior to introducing the at least onesubstantially well-developed flow stream of the second gas into themixing chamber, and wherein the at least one substantiallywell-developed flow stream of the second gas is more than one.
 21. Themethod of claim 18, further including: using an engine to provide thesecond gas and sampling the combined stream in the secondary mixingregion for compliance with emission standards.
 22. A mixing chamber formixing a first stream of gas with a second stream of gas, comprising: aninternal volume defined by a first end, a second end, and wallsextending between the first and second ends, the second end having agradually converging portion, such that no structure extends from thefirst end, from the second end, and from the walls into the internalvolume; a first inlet opening configured to receive the first stream ofgas into the mixing chamber, the first inlet opening located at thefirst end; a plurality of second inlet openings configured to receivethe second stream of gas into the mixing chamber, the plurality ofsecond inlet openings symmetrically positioned with respect to the firstinlet opening; and an exit opening configured to discharge a combinedstream of gas formed from the first and second streams of gas from themixing chamber, the exit opening located downstream of the graduallyconverging portion.
 23. The mixing chamber of claim 22, wherein thefirst stream of gas is a stream of exhaust gas from an engine and thesecond stream of gas is a stream of dilution air.
 24. The mixing chamberof claim 22, wherein the first end, the second end, and the walls haveinternal surfaces formed of electro-polished, passivated, stainlesssteel.